The need for more precise delivery mechanisms for analytical chemistry techniques is increasing. Potentially, syringe pumps offer the resolution and dynamic range of an analytical balance for the delivery of liquid reagents. Unfortunately, prior-art syringe pump designs do not achieve this precision, resolution and dynamic range.
Most syringe pumps use a stepping motor to turn a lead screw or its mating nut to drive the syringe plunger. The lead screw/stepping motor combination allows for a simple, digitally-controlled open-loop plunger drive mechanism having a resolution defined by the number of steps per revolution of the stepping motor and the pitch of the lead screw. By simple calculation it can be determined that a linear resolution as fine as 94 nm per step can be achieved with a lead screw pitch of 1.2 mm per revolution and a a micro-stepping motor having 12,800 steps per revolution. A mechanism of this type driving the plunger of a 5 ml Hamilton Gastight syringe, can theoretically achieve a liquid delivery resolution of 10 nl. Current syringe pump designs compromise the precision and repeatability of this resolution in their mechanical architecture and layout.
Many syringe pumps have the axis of the lead screw offset from the axis of the syringe plunger. Such an offset drive may cause flexing of links that connect the plunger to the lead screw, warping of the plunger itself and/or warping of the syringe barrel, resulting in inaccurate deliveries and leakage. These designs also have substantial hysteresis, making them unusable for precision infusion/extraction applications.
Some designs have the lead screw axis aligned with the axis of the syringe plunger, turning either the screw or the nut through a gear train. Gear trains are used to increase drive torque to compensate for smaller motors. They also allow for the use of an off-axis motor creating a more compact design, and they increase the effective resolution of a low resolution stepping motor. Although the problem of backlash of gear trains can be eliminated, gear trains introduce additional errors to the motion of the plunger and add complexity to the overall mechanical design. Other designs have the lead screw axis offset from the axis of the syringe plunger and use a gear train to couple the lead screw drive to the plunger, which necessarily compromises accuracy.
The principal objectives of this invention are:
1. To provide a coaxially driven syringe pump with all forces aligned with the plunger's axis of motion. The essential elements that are axially aligned are the syringe barrel, the syringe plunger, the means for coupling axial motion of a lead-screw drive mechanism to the syringe plunger, the axis of the lead screw/nut assembly and the drive motor (preferably a stepping motor). PA1 2. To provide a highly repeatable drive mechanism. A repeatable system can be calibrated and made highly accurate. PA1 3. To allow for misalignment of elements due to errors in manufacturing and/or assembly and still provide high repeatability.
In the preferred embodiment of this invention, the errors are limited to the precision of the drive motor (which may be a servo or other motor, preferably a stepping motor), the precision of the lead screw, the thermal expansion coefficients of the mechanical components and the thermal expansion coefficients of the liquid being pumped. Any error due to compressibility of the materials used for the drive system should be several orders of magnitude less than the combination of the above factors. Only the compressibility of the plunger head in the syringe barrel is of any significance because it necessarily includes at least one ring of compliant material for sealing between the plunger and the wall of the syringe barrel. By limiting the errors of the mechanism to those previously described, a simple open loop control system can use a stepping motor to obtain highly accurate results.
A rigid body has six independent degrees of freedom for motion. They are translational motion along three orthogonal axes and rotational motion about each of the three orthogonal axes. The definition of "kinematic support" requires that, for every degree of freedom to be constrained between two bodies, there be only one point of contact between them. If more than one point of contact exists per degree of freedom, the system is said to be degenerate. In a semikinematic design, a point contact may be expanded to a surface in order to bear a larger load, provided that the contact may nevertheless be theoretically reduced to a point. However, where a semikinematic coupling is used, it is necessary that the contacting surfaces be "run in" to insure proper alignment by operating the coupled mechanism through its full range of expected motion a sufficient number of times for the parts in contact to wear off "high spots" on the contacting surfaces before final instrument alignment. Thus, through kinematic design or at least semikinematic design, repeatable motion can be achieved with precision without relying on precision manufacture of the parts. If the motion is repeatable, it can be measured and calibrated and thus made highly accurate.